From Xojo Documentation
The time (in milliseconds) between executions.
Periods of less than or equal to zero default to a value of 1 millisecond. The default value is 1000. The rate that a Timer can actually fire depends on the speed of the host computer, the operating system, and other tasks the computer is doing. It is possible to set Period to a value that cannot be achieved by the computer that is running the application.
This example (in the Timer.Open event) sets the Period to 500 milliseconds.